The eukaryotic cell

The eukaryotic type cell can be schematically divided into three main parts: the nucleus, the cytoplasm and a complex of membranes; in the cytoplasm there are also several other organelles.

Dimensions and Cell Forms

Most cells that make up the plant, or animal. it has diameters between 10 and 30 micrometers. The main limitation to cell size seems to be due to the relations between volume and surface. The materials that enter and leave the cell must pass through the surface and the more active a cell is, the faster these materials must pass. Furthermore oxygen, carbon dioxide and other metabolically important molecules enter and leave the cell by diffusion, which is effective over short distances. The materials can move faster in, out and through the small cells.

It is therefore not surprising that the most metabolically active cells are usually small. The relationships between cell size and metabolic activity are well illustrated by egg cells. Many egg cells are very large; a frog egg for example has a diameter of 1500 micrometers; other egg cells have a diameter of several centimeters (largely due to the reserve material). When the egg cell is fertilized and begins to be metabolically active, it divides several times before any other increase in volume, or mass, thus dividing its cellular unit into metabolically more efficient dimensions.

A second limitation to cellular size appears to be the ability of the nucleus to regulate the activities of a large metabolically active cell. Furthermore the exceptions seem to confirm the rule: in certain large and complex unicellunar animals, the ciliates (of which the paramecium represents an example), each cell has two or more nuclei, and those in more seem to be copies of the original.

As with water drops and soap bubbles, cells tend to be spherical; they take different forms due to the cell walls (as happens in most plant cells and in many single-celled organisms), or following adhesion and pressure exerted by other cells or nearby surfaces.